The ATSC digital television standard presently provides for the transmission of successive data fields each comprising 313 segments extending over a 24.2 ms time interval. FIG. 1 discloses an exemplary format for a data field according to this standard. The first segment of each field is a field sync segment. The field sync segment is composed of four two-level segment sync symbols and space for 828 other two-level symbols. A portion of this space is used for a field sync, and another portion of this field is reserved. Except for the reserved portion, the information in the frame sync segment does not change from field to field. Each of the remaining segments of each field comprises four two-level segment sync symbols and 828 n-level data symbols where n is currently eight, although n could be other integers such as two, four, sixteen, etc. Except for the segment sync portion, it is highly likely that the data in the remaining segments of the fields change from field to field.
As indicated by U.S. patent application Ser. No. 09/804,262 filed on Mar. 13, 2001, there is presently some interest in extending the ATSC digital television standard to allow a field to contain a mix of more robustly coded data (referred to herein as E-VSB data) and the data currently provided for in the standard (referred to herein as VSB data). Preferably, the data mix is employed on a segment-by-segment basis such that some segments of a field are used to transmit VSB data exclusively and the remaining segments of the field are used to transmit E-VSB segments exclusively. However, it is possible that all data segments of a field could contain either E-VSB data segments exclusively or VSB data segments exclusively. Moreover, it is also possible that the E-VSB data contained in some segments of a field may be coded with one robust coding rate and that the E-VSB data in other segments of the field may be coded at other robust coding rates.
As disclosed in the above mentioned '262 application, a map that indicates which segments contain the more robust (E-VSB) data and which segments contain standard VSB data is preferably provided by the transmitter to the receiver so that the receiver can properly decode and otherwise process the received VSB and E-VSB data. Assuming that a field contains E-VSB data at different coding rates, the map in that case must also designate the coding rates that apply to the differently coded E-VSB data segments.
The '262 application describes one mapping system. Co-pending U.S. patent application Ser. No. 10/011,900 filed Dec. 3, 2001 as well as the '333 application describe another mapping system that reliably identifies which segments contain first data (such as VSB data) and which segments contain second data (such as E-VSB data).
Multipath distortion, commonly found on terrestrial television channels, can affect the ability of the receiver to properly receive and process the map. For example, in the case of map data transmitted in the reserved portion of the field sync segment, data that tends to be random from field to field will be superimposed on the map data if a ghost is a long ghost such that it occurs in the data of a data segment rather than in the field sync segment. If the map and its duplicate are transmitted in two successive field sync segments, the map and its duplicate add with a high degree of correlation when a map and its duplicate are averaged in a receiver, but the superimposed data add with a much lower degree of correlation. Thus, the map is more easily distinguished from the data. Accordingly, the map is readily detectable.
On the other hand, if the ghost is a short ghost such that it occurs in the frame sync segment of a field, frame sync symbols that may not vary from field to field are superimposed on both the map and the duplicate. Accordingly, while the map and its duplicate add with a high degree of correlation, the superimposed field sync symbols also add with a high degree of correlation. Thus, the map cannot be easily distinguished from the field sync symbols. Accordingly, the map is difficult to detect.
As indicated in the aforementioned '394 application, interleaving is used to minimize the adverse effects of burst noise. However, as discussed more fully below, there is a latency that is inherent from the interleaving of the map that is used to indicate where in a frame the receiver can find the various data. This latency results because any one map, due to the interleaving, is spread out over a number of frames so that the map cannot be completely received until all of the frames over which it is spread are received. As a result, the latency associated with interleaving significantly increases receiver acquisition time.
Moreover, while interleaving generally protects against a noise burst impairing all of the map bits or symbols in a particular field, the effectiveness of such protection is reduced in the event of multiple noise bursts in that or subsequent fields. Repeated noise bursts could prevent effective reception of the map even though the robust and other data in the frame are received without appreciable degradation.
The present invention, in one of its embodiments, allows maps to be more easily detected even in the presence of noise bursts.